Mac newbie question: Saved images too bright

I've recently moved from a PC to a Mac and from CS2 to CS3 version of
Photoshop. I've noticed that after doing this, when I save an image
out from Photoshop the result looks too bright when I view an image in
any image viewer or web browser on the Mac, but the colours look as
they were intended when reloading the image in Photoshop again. I've
tried various settings for colour profiles (simply setting "Don't
colour manage this file" on Windows produced the expected result but
doesn't seem to work on the Mac), tried saving the file both with and
without the embedded colour profile, but nothing I try seems to
work.

This is getting very frustrating as I mostly produce images for web
consumption and need to be able to RGB match colours precisely so that
for example the image's background colour matches against a CSS
specified background colour. I tried searching Google and in this
group but couldn't find anything that looked useful. I'm surely not
the only person to have run into this problem, can anyone help?

I do not have a solution for you but I admit I do not clearly understand
what you are describing.
If you have created an RGB specified background color that matches the RGB
analyzed object then these should display the same regardless of program,
although they may not appear to have the same tonal values when viewed
outside of Photoshop. That is the same in both Apple and Windows OSes.
You are likely aware that monitor calibrated color management is intended
for reasonably predictable WYSIWYG color printing. That is the same in both
Apple and Windows OSes.
I presume you are aware that using other than dedicated graphics panels one
frequently encounters the problem that color managed output, meaning prints,
are darker than what is perceived on overly bright and not really
adjustable LCD panels. Apple panels are consumer grade and subject to this
like nearly all others. More than some, in fact.
If color matched printing is not your object but you want images to appear
more or less similar in all programs it has been recommended to use Monitor
RGB as your color space in Photoshop.
While some variation is common when viewing an image outside a color managed
Photoshop environment it should not be all that extreme in either the Mac OS
or Windows-- even the abomination that is Vista. It could be that your color
management settings in Photoshop may not be correct. Perhaps you are not
creating your images in the appropriate color space to begin with.